Best way is to go through a booking agent if you have one. Otherwise in past years Red Stripe have done a showcase type event for local bands and the 'best' one usually gets a slot on the Saturday, details of that usually appear on the festival website a few months before. Alternatively you can fill this form in: http://escapegreat.com/artistRegistration/

probably best for you to hold off and see what route they offer to local bands this year or you could try for one of the fringe events, they're usually in the day time... booking agents are notoriously hard to get.